Did not compile without structopt crate

[genya0407]

Projects using argopt did not compile.

Reproduction

$ mkdir reproduction
$ cd reproduction 
$ cargo init      
     Created binary (application) package
$ rustc --version
rustc 1.43.0 (4fb7144ed 2020-04-20)
$ echo 'argopt = "0.1.1"' >> Cargo.toml 
$ cat Cargo.toml            
[package]
name = "reproduction"
version = "0.1.0"
authors = ["Yusuke Sangenya <[email protected]>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
argopt = "0.1.1"

$ cat > src/main.rs <<SRC
use argopt::cmd;

#[cmd]
fn main(
    #[opt(short = "b", long = "bin")] bin: Option<f64>,
    #[opt(short = "w", long = "screen-width", default_value = "80")] screen_width: i64,
) {
    println!("{:?}", bin);
    println!("{}", screen_width);
}
SRC

$ cargo build            
    Updating crates.io index
   Compiling proc-macro2 v1.0.21
   Compiling unicode-xid v0.2.1
   Compiling syn v1.0.41
   Compiling version_check v0.9.2
   Compiling libc v0.2.77
   Compiling fnv v1.0.7
   Compiling strsim v0.9.3
   Compiling bitflags v1.2.1
   Compiling ident_case v1.0.1
   Compiling unicode-width v0.1.8
   Compiling unicode-segmentation v1.6.0
   Compiling strsim v0.8.0
   Compiling vec_map v0.8.2
   Compiling ansi_term v0.11.0
   Compiling lazy_static v1.4.0
   Compiling textwrap v0.11.0
   Compiling heck v0.3.1
   Compiling proc-macro-error-attr v1.0.4
   Compiling proc-macro-error v1.0.4
   Compiling quote v1.0.7
   Compiling atty v0.2.14
   Compiling clap v2.33.3
   Compiling darling_core v0.10.2
   Compiling structopt-derive v0.4.10
   Compiling darling_macro v0.10.2
   Compiling darling v0.10.2
   Compiling argopt-impl v0.1.0
   Compiling structopt v0.3.17
   Compiling argopt v0.1.1
   Compiling reproduction v0.1.0 (/home/sangenya/dev/reproduction)
error[E0433]: failed to resolve: could not find `structopt` in `{{root}}`
 --> src/main.rs:3:1
  |
3 | #[cmd]
  | ^^^^^^ could not find `structopt` in `{{root}}`
  |
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: could not find `structopt` in `{{root}}`
 --> src/main.rs:3:1
  |
3 | #[cmd]
  | ^^^^^^ could not find `structopt` in `{{root}}`
  |
  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0433`.
error: could not compile `reproduction`.

To learn more, run the command again with --verbose.

Workaround

Adding structopt to dependency solved this issue.

$ echo 'structopt = "*"' >> Cargo.toml 
$ cargo build
   Compiling reproduction v0.1.0 (/home/sangenya/dev/reproduction)
    Finished dev [unoptimized + debuginfo] target(s) in 0.73s